HomeMy WebLinkAboutMulti-Hazard Mitigation Plan: 13. Coastal Erosion CIVIL DEFENSE AGENCY
COUNTY OF HAWAII
920 ULULANI STREET HILO,HAWAII 96720
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13. Coastal Erosion
Chapter 13:Hazard Analysis—Coastal Erosion
CHAPTER 13 - COASTAL EROSION
13.1 Description of Hazard
Beach erosion and coastal erosion are not the same,but they are related. Beach erosion is a
reduction in the amount of sand a particular beach has. On a global level, sea level rise
causes beach erosion. But beaches also erode(and expand)on a seasonal basis.
Beaches get sand from both the ocean and the land. Larger waves move sand from the coastal
sand dunes off into the ocean. This raises the seafloor, flattens the overall profile of the
beach, and,therefore, causes waves to break further offshore. This, in turn,minimizes the
waves'impact on coastal lands. Beaches recover from these seasonal shifts when the waves
move the sand back onto the beach and the winds blow the deposited sand into dunes. These
dunes will store the land-based sand until the next large wave event.
Initial shore prof lie.
—width—
Shnrelirre profile after retreat.
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Aneach undergoing net longierm sweat will
*nainlain its natural width.
Inital shore praflie.
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Snswuq Stloreline profile after retreat.
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Bench loss eventually nccurs in trorit r)f a seHwall
trrr a t}eaCh experiencirxl net InrHyerm retreat.
Figure 13-1. The impacts of stabilization on shoreline retreat and beach loss
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Chapter 13:Hazard Analysis—Coastal Erosion
Coastal erosion occurs when the beach migrates toward the land in order to compensate for
beach erosion as it tries to maintain a constant supply of sand(see the right side of the
photo). If sand is not available to a beach, such as when a wall is built to protect the land,the
land is stabilized,however beach erosion will occur(see left side of photo).
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Unstabilized -
Stabilized - land lost
beach lost beach preserved
land preserved
Figure 13-2. Effects of beach stabilization
Photo courtesy of Charles Fletcher
Installing a seawall or revetment(i.e.,hardening a shoreline) interferes with the natural cycle
of beach erosion. Rather than pulling sand from a landward supply in order to promote waves
breaking further off-shore during the seasonal high wave period, the seawall or revetment
prevents this natural phenomena from occurring. Thus,the land itself begins to erode.
Therefore, it is tragically ironic seawalls or revetments have been installed to prevent coastal
erosion, but their very presence exacerbates the very problem they were supposed to resolve.
(Source: Fletcher, Charles, Eric Grossman, Bruce Richmond. Atlas of Natural Hazards in the
Hawaiian Coastal Zone. 2000.)
13.2 Significant Historical Events
Due to the youthful geographic age of Hawaii Island, sandy beaches are fairly uncommon;
instead rocks or cliffs line most shores.
13.3 Probability of Occurrence
Sandy beaches are dynamic sedimentary systems that naturally experience phases of erosion
and accretion that operate over a range of time intervals. Frequent short-term changes are
seasonal - erosion mostly occurs in seasons when storms that generate erosional wave
regimes are more frequent. Rapid erosional episodes may also be produced by high-
magnitude storms, such as tropical cyclones or intense low pressure systems. The degree of
erosion that occurs within a particular erosional phase can be highly variable, and this
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Chapter 13:Hazard Analysis—Coastal Erosion
likewise is linked to the magnitude and frequency of storms that impact on the coast. For
example, during a 100 year high magnitude storm, waves may erode several meters into the
foredune that sits well behind the normally active zone of accretion and erosion. Also,
several lower-magnitude storms that occur in quick succession can produce a similar degree
of erosion because the intervening periods are too short for constructive swell waves to push
a significant amount of sediment back to the shoreline.
The accretion of sand on beaches occurs during the more quiescent seasons when average
swell waves deliver sediment back to the shoreline. Beach accretion, however, is generally a
much slower process than beach erosion. For example, it may take several years for a beach
to return to its pre-storm condition after one major storm or several smaller storms in quick
succession.
13.4 Risk Assessment
Since this is not a major hazard in Hawaii County,no method of assessing the economic risk
due to coastal erosion has been utilized for this plan.
Of greater concern than coastal erosion in Hawaii County is the potential for sea-cliff
erosion. Homes perched upon these cliffs may be at risk if significant erosion does occur.
There is no established recurrence interval for such events and early warning signs would
allow ample time to mitigate any catastrophic failures.
13.5 Mitigation Strategies
Below are several potential coastal mitigation measures that have been applied with varying
degrees of success in other coastal regions. In the event that mitigation becomes necessary in
Hawaii County the validity of each of these measures should be assessed in greater detail.
13.5.1 Revise the Shoreline Setbacks
The intent of shoreline setbacks is to establish a coastal-hazard buffer zone to protect beach-
front development from coastal erosion. Adequate setbacks allow the natural erosion and
accretion cycles to occur and help maintain lateral beach access. Furthermore, setbacks
provide open space for the enjoyment of the natural shoreline environment.
A 40-foot shoreline setback has been often inadequate because they are not determined by
historical site-specific rates of coastal erosion. Coastal-erosion hazard maps could be used to
establish rate-based building setbacks. Setbacks would be site-specific in order to reflect the
nature of coastal erosion. These setbacks would also incorporate the proposed style of
development. Construction-style considerations would include the size and expected lifetime
of the planned structure. Larger, immovable buildings and those with lifetimes of more than
50 years would have deeper setbacks than small,movable structures.
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Chapter 13:Hazard Analysis—Coastal Erosion
13.5.2 Utilize Inter-agency communication
Better cooperation between State Parks and the Counties' Parks and Recreation should be
sought since these departments maintain many of our beach parks. In addition, discussions
should be enhanced with the State Department of Transportation (DOT), because some
highway facilities are threatened or will soon be threatened by coastal erosion. In most cases,
state highway facilities are the major or only thoroughfare between regions. Proactive
planning of new highway construction, widening, and realignment could reduce the threat of
coastal erosion and beach loss by anticipating future shoreline trends.
13.5.3 Increase Public Awareness
Coastal erosion is largely promulgated by the activities of humankind. Increasing public
awareness of the importance of seasonal beach erosion, and how their activities can
exacerbate coastal erosion,becomes extremely important.
13.5.4 Promote Research Projects
Projects that focus on coastal processes can lead to improved erosion management. Further
research, along with beach monitoring, is necessary to increase our understanding of coastal
and marine science and to insure effective and efficient management of the coastal zone.
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